Abstract: A pseudostationary phase of organic solvent was produced from the wetting of hydrophobic tubing by the solvent within the concurrent flow of two immiscible phases through narrow tubing. The phase retention differences in a hexanol/aqueous acetate buffer of pH 4 (4:1) segmented flow system were used to retard extracted o-nitrophenol as test analyte and then concentrate it with a single 20 µL injection of 0.1 M NaOH as eluting reagent. Detection was carried out at 247 nm. The effects of sample volume, extraction coil length, flow rate, phase ratio and tubing diameter on the pre-concentration factor were studied. A throughput of 30 samples/h and pre-concentration factors in excess of 50-fold were obtained using a 900 cm long, 1 mm i.d. coil and a flow rate of 1 ml/min. In this work, a pseudostationary phase of organic solvent replaces the microcolumn in flow injection pre-concentration. This pseudostationary phase results from the wetting of hydrophobic tubing by the organic solvent within the concurrent flow of two immiscible phases through narrow tubing. If this wetting film is sufficiently thick, a differential velocity between the aqueous and organic phases develops within the two-phase segmented flow. The effect of experimental parameters such as sample volume, flow rate, phase ratio, and tubing diameter on the pre-concentration factor are characterized. Using these procedures, pre-concentration factors in excess of 50-fold are obtained with a throughput of 30 samples/h. Copyright 1995, American Chemical Society.